This invention relates to an improved rotational vibratory viscometer transducer and circuit, for measuring the viscosity of fluids.
Rotational vibratory viscometers are well known in the art, and generally comprise (i) a transducer having a tip immersible in a fluid the viscosity of which is to be determined, (ii) an electromagnetic drive coil for causing the tip to rotationally oscillate with a very small angular amplitude, (ii) a feedback control circuit for maintaining the angular amplitude of oscillation of the tip at a predetermined constant value irrespective of the viscosity of the fluid, and (iv) a circuit for determining the power supplied to the drive coil, usually by squaring the current supplied to said coil, which power is a measure of the viscosity of the fluid.
A viscometer of this type is described, for example, in "Viscometer for Energy Saving", J. V. Fitzgerald, F. J. Matusik, and P. C. Scarna, Jr., Measurements & Control, April 1980. Similar viscometers are described in the references cited in said article, as well as in U.S. Pat. Nos. 3,382,706; 3,710,614; 3,712,117; 3,762,429; 3,875,791; and 4,299,119.
In the commercial use of such viscometers difficulties have been encountered in the manufacture of the transducers and in the stability of the zero adjustment of the instruments.
In manufacturing transducers of the type shown, for example, in FIGS. 1 and 2 of U.S. Pat. No. 3,382,706, approximately 50% of the transducers had to be discarded or remanufactured because they would not oscillate consistently in conjunction with the associated control circuit. The vibratory characteristics of the transducers varied widely from one unit to the next, although all units were made with the same materials and dimensions.
In using those transducers which oscillate properly, the usual procedure is to calibrate the instrument by setting the output thereof to zero with the tip oscillating in air or vacuum (to compensate for internal energy losses in the transducer). However, this zero setting tended to drift with time at a troublesome rate, and to vary substantially with temperature, necessitating frequent readjustment of the instrument (typically at intervals on the order of 15 minutes). As a result, such instruments have been virtually unusable for remote on-line monitoring of the viscosity of fluids in process control applications.
Accordingly, an object of the present invention is to provide an improved rotary vibratory viscometer transducer exhibiting greater manufacturability and operational stability than similar prior art transducers, and an improved circuit for use in conjuction therewith.